The present invention relates generally to electrochemical measuring systems and relates more particularly to an electrode assembly for such a system. The present electrode assembly includes an electrode member adapted for introduction into the bore of a measuring chamber and having a selectively sensitive membrane disposed in its front end and an electrical conductor extending from the membrane rearwardly through the electrode member. A holding member which is engageable with the element defining the measuring chamber bore is provided for securing the electrode member in position within the bore.
In a known system of this type (German Offenlegungsschrift No. 2,212,801) the electrode member which carries the selectively sensitive membrane is inserted in a bore on the lower end of which are located supply and discharge channels for the sample to be analysed, so that said samples can be brought into contact with the membrane. A cup-shaped holding member is threadedly attached to the rear end of the bore wall extended in a tubular manner for securing the electrode member in the bore and for sealing purposes. In addition, an O-ring is placed between the holding member and the electrode member to provide a seal. The conductive connection is passed out of the electrode member through the base wall of the holding member.
With such a system due to the rotary movement required to attach the threaded holding member, there is a corresponding rotary movement of the electrode member about its longitudinal axis with the danger that the very sensitive membrane may engage the peripheral areas of the supply and discharge channels at the front end of the electrode member resulting in damage thereto. Furthermore, the holding force acting in the longitudinal direction on the electrode member can only be accurately adjusted with considerable difficulty due to the threaded attachment of the holding member, so that there is either a risk of leaks or of the electrode member being damaged.
It is also known (German Offenlegungsschrift No. 2,726,772) to construct the electrode member from a plastic member carrying the membrane and which is firmly connected with a rear metal mounting having an external thread for screwing into the bore wall, as well as a contact to which the electrical conductor is connected.
Quite apart from the fact that this known arrangement is complicated and costly to manufacture, the difficulty once again arises that a rotary movement is necessary for mounting the electrode member which can damage the membrane while the force acting in the axial direction of the secured electrode member can only be gauged with great difficulty, so that the electrode member is frequently either secured too firmly or too loosely in the bore wall, which can impair the sealing effect. Finally, in these known electrode systems where polytetrafluoroethylene (PTFE) is used as the plastics material for the electrode member, under the action of mechanical pressure the plastic exhibits flow phenomena, so that after a certain time there is a deterioration in the seal between the front end face of the electrode member and the measuring chamber, making it necessary to subsequently regularly adjust the screw connection.